1. Field of the Invention
This invention relates to a method of producing coagulated colloidal particles with close-packed structure by flocculating or coagulating aqueous colloidal particles to form particles with regularly packed structure and recovering the particles thus separated from the colloidal dispersion.
Aqueous colloidal particles herein means dispersion systems comprising water as dispersion medium and solid or gel particles, 10 to 3000 .ANG. in size, as disperse phase, said disperse phase being electrically charged, suspended in said aqueous dispersion medium, and capable of being flocculated or coagulated by addition of a coagulant. It goes without saying that polymer latexes obtained by emulsion polymerization or other methods fall under this category.
2. Description of the Prior Art
To recover the solid or gel particles from such aqueous colloidal dispersions, both natural and synthetic, it is common practice that the primary particles, which are extremely minute in size, are first collected into a mass and the agglomerate thus formed is recovered, purified and dried. This technique is widely used for a variety of colloidal dispersions, but often meets with difficulties when applied to polymer (synthetic resin) latexes (most popular aqueous colloidal particles) because of the low softening point of disperse phase.
Recovery of a resinous polymer from a latex containing the same is generally effected by adding an aqueous solution of coagulant, such as inorganic salts or acids, to the latex (or adding the latex to the aqueous solution of coagulant), to cause coagulation, thus forming a slurry by heating or other suitable treatment, dewatering and drying the resulting slurry, and collecting the coagulated particles in the form of powder. This method, however, has the disadvantages that the powder obtained is irregular in shape, its particle size is difficult to control, particle size distribution is broad, and a considerable quantity of fine powder is contained. The result is airborne loss of the product, frequent troubles encountered during manufacturing due to clogging with fine powder, adversely affected working conditions due to dust formation, increased risk of dust explosion, and others. In addition, it is hardly possible to produce powder of high bulk density, resulting in higher transportation costs and larger warehouse expenses; and high-cost dewatering and drying facilities are required because of the poor dewatering, drying, flowing and anti-blocking properties of the powder obtained.
Tremendous efforts have been devoted to overcome these difficulties in the synthetic resin manufacturing industry, and many techniques have been proposed. These include a method of recovering the polymer in the form of spherical particles by spraying a polymer latex into a coagulatory atmosphere, and a method of absorbing coagulated particles by an organic solvent dispersed in water, followed by removal of the solvent to cause solidification of the particles. Behind these efforts to develop new methods for recovering polymers from their latexes, there is the current tendency that the value of a polymer product tends to be dictated by the particle size and its distribution, packing rate and related powder characteristics, rather than by the basic properties of that polymer.
The former method mentioned above has the disadvantages that it is difficult to produce, on an industrial basis, particles larger than 500 .mu.m in size and that particles of close-packed structure can hardly be obtained. The latter method also involves various problems in terms of cost, quality, scope of application and powder characteristics. These include the difficulty in removing solvent and foreign matters and in achieving particles of close-packed structure, and the difficulty, for some types of polymers, in finding a solvent that can be used in such a special system.
Spray drying and vacuum agglomerate drying are known as a means to obtain solid polymer particles by direct drying of latices, and many modifications and improvements of equipment have been proposed. The disadvantages of these methods are that components other than the polymer contained in the latex can hardly be removed, that large equipment is required for production of large particles, and that a vast quantity of energy is necessary for treatment of low-concentration latices. This means limited use of these methods in terms of product quality and manufacturing cost. What is common to all these conventional techniques is difficulty in removing impurities and the problem of poor quality of emulsion polymers is left still unsolved.